Conventional game racquets for playing tennis, racquetball, squash or other games have their playing surfaces formed by opposite sides of a single interwoven, tensioned string bed lying in a central plane of the head frame of the racquet and spanning the central opening bounded by the head frame.
Double-strung racquets, that is racquets which have their playing surfaces formed by mutually parallel separate sets of interwoven tensioned strings in planes bounded by the outer edges of the head frame, have been proposed.
While the advantages of double-strung racquets are well known, it has previously been impractical to produce them in a cost effective and efficient manner.
In one technique of making double-strung racquets, as disclosed in U.S. Pat. No. 4,320,900, the racquets require a specially made frame with holes and grooves formed in the outside edges. This type of racquet requires a large number of strings, one for each vertical and horizontal line or component of the playing surfaces. Each string must be knotted at both ends, strings of different lengths are required, and a special tool is needed to thread the strings into the holes and grooves. Another disadvantage of a racquet of this type is that the strings soon lose their original tension, no matter how they are anchored or knotted.
In another technique of making double-strung racquets, as disclosed in U.S. Pat. No. 3,968,966, a specially designed frame including an elongated band with a T-shaped cross section is used. Grooves are cut into the outside edges of the frame and two single strings are wrapped around the outside edges of the frame to form the required two parallel surfaces. There is no interaction between the strings of each surface.
The racquet of this second type suffers from a number problems and disadvantages, both in its manufacture and use.
First, the T-shaped cross section increases the wind resistance of the racquet. In general, the head frame of a racquet should be streamlined to permit the free flow of air around the racquet. A racquet design that impedes this free air flow is unsuitable for serious game playing.
Second, by placing the strings in grooves on the outside edges of the frame, the frame can come into direct contact with a ball, thereby causing the ball to be misdirected when struck. When a ball comes into contact with the inner edge of the frame of a conventional racquet, it is ineffective in controlling the direction of the ball.
Third, using two independent strings to create the two parallel surfaces, one string for each surface, can easily create uneven pulling and distortion of the frame while it is being strung. To avoid this problem it would be necessary to use exactly the same tension on the top and bottom strings simultaneously.
A fourth disadvantage of having two independently strung playing surfaces is that the faces vibrate unevenly when a ball is struck. Thus the frame as a whole does not absorb the shock evenly.
It is desirable to produce a double-strung racquet which uses a conventional frame, does not require special strings or tools to string, and is easy to make. It is also desirable to produce such a racquet without increasing the wind resistance of the racquet. It is further desirable to have a double-strung racquet with playing surfaces as large as the outside circumference of the head frame, and not one in which the size of the playing surfaces has been reduced.
It is further desirable to produce a double-strung racquet which can be strung without uneven pulling of the head frame.